ABSTRACT
Besides the template method, templateless epitaxy methods have also been extensively explored to synthesize inorganic semiconductor nanoarrays. The epitaxy methods include vapor-phase epitaxy and liquid-phase epitaxy. The vapor-phase epitaxy method requires a lattice-matched substrate, on which oriented nanoarrays could grow, with the orientation being controlled by the lattice-matching relation. Because it is not always possible to find a suitable substrate at a reasonable cost, vapor-phase epitaxy thus has limitations in spite of the good crystalline quality of the nanoarrays by this method. Liquid-phase epitaxy, on the contrary, does not need lattice-matched substrates; instead, a seed layer could be used to direct the orientational growth of the nanoarrays on almost any substrates at reasonably low temperatures. The liquid-phase growth does not need any expensive facilities and is ideal for scale-up production, although the crystalline quality is poorer than in vapor-phase epitaxy, which generally involves a high-temperature growth environment. Another well-adopted method is the anodization and etching method. In the anodization method, a metal foil is anodized in an acidic electrolyte. A native oxide usually exists on the surface of the metal foil, which is etched in the electrolyte and pitting is formed. The subsequent anodization forms pores, and oxide nanotube arrays develop with the process of anodization. For metals of Ti, Zr, and Ta, a nanotube array could be synthesized by this method readily. The PAAM mentioned in the previous paragraph is also produced by the anodization method, where nanopore arrays instead of nanotube arrays are formed. Electroless etching of a Si wafer has also been used to synthesize Si nanowire arrays. In the method, pores are formed by chemical etching using hydrofluoric acid, and further lateral etching transforms the pore arrays to nanowire arrays. Similar to etching a Si wafer, gas molecules or ions in solution could directly etch and react with a metal foil and transform the surface layer of the metal foil to nanoarrays. In the following sections, these synthesis methods will be elaborated. 10.2.2 The Template MethodIn the template method, nanoarrays are deposited in the pores of the PAAM template mainly via five routes, that is, electrodeposition,
electroless deposition, electrophoretic deposition, the sol-gel soaking method, and chemical vapor deposition (CVD). 10.2.2.1 The electrodeposition methodIn the electrodeposition method, the template is pressed on a metal electrode (working electrode) and positioned in an electrolyte containing the ions and molecules of the growing materials, with a graphite plate as the counterelectrode. A standard electrode, such as a saturated Ag/AgCl electrode, is used as the reference electrode. A voltage is applied across the working electrode and the reference electrode. Usually, to control the stoichiometry of a semiconductor material in the electrodeposition, an optimum pH value should be specified and a complexing agent should be added to adjust the concentration of ions. For example, Peng et al. investigated the electrodeposition of CdSe nanowire arrays in PAAM templates [20]. The electrolytes they used were composed of 0.1 M CdCl2 and 0.2 mM SeO2 as the source materials and 0.2 M Na2SO4 as the supporting electrolyte. The pH value of the electrolyte solution was adjusted to ~9 by adding ammonia solution. CdSe was deposited at –0.9 V versus the Ag/AgCl reference electrode at room temperature for 12 hours. The top and cross-sectional views of CdSe nanowire arrays are displayed in the scanning electron microscopy (SEM) images in Fig. 10.1a, b, respectively. From thermodynamic calculations, they found that a pH value of 9 was the optimum for stoichiometry of the compound, which was consistent to the experimental results where a low pH value favored the deposition of Se-rich compounds and a high pH value, the deposition of Cd-rich compounds. In addition, to obtain a stoichiometric compound, the Cd and Se deposition reaction should proceed at a similar overpotential; therefore, a complexing agent, ammonia, was used to control the concentration of Cd ions and Se ions and the reduction potentials. Figure 10.1c shows the X-ray photoelectron spectroscopy (XPS) spectra of Se and Cd in the deposited CdSe nanowires, where fitting of the data gives a stoichiometric compound of CdSe. A wide variety of semiconductor nanowire arrays have been synthesized by electrodeposition methods using PAAM as the template, such as ZnO, CuInSe2, CdS, V2O5, and Bi2Te3 [21-25]. As a typical example, Zheng et al. reported the direct deposition of ZnO nanowire arrays in the template in 2001 [21]. The deposition was
carried out in a three-electrode cell in a 0.1 M zinc nitrate solution at 343 K at 1 V versus the Ag/AgCl reference electrode, with a zinc sheet as the counterelectrode. As shown in Fig. 10.2, ZnO nanowires are in ordered arrays in the SEM image, and the nanowires are single crystals in the transmission electron microscopy (TEM) image and the selected area electron diffraction (SAED) pattern. This direct deposition has the advantage that the oxide nanowires are single crystalline, whereas, as will be introduced in the following sections, those nanowires synthesized by chemical transformation from metal nanowires tend to be polycrystalline.
